The present invention relates to an X-ray exposure apparatus used for manufacturing semiconductor integrated circuits, and particularly to an X-ray exposure apparatus suitable for controlling a supply of a low X-ray absorbing gas to be filled in a less-attenuation chamber interposed between an X-ray source and a mask.
If atmospheric air exists in an X-ray transmitting path between an X-ray source and a mask in an X-ray exposure apparatus, X-rays are disadvantageously absorbed in the atmospheric air, resulting in low transmittivity of the X-rays, under-exposure to the X-rays, lower through-put and less accuracy in alignment. Therefore, in the prior art, a less-attenuation chamber is provided between an X-ray source and a mask, and the inside of the chamber is made a vacuum or filled with a low X-ray absorbing gas, for example, He (helium), i.e. a gas having a small absorption coefficient for X-rays which absorption coefficient for X-rays is smaller than that of atmospheric air. As one of the prior art disclosed, "IEEE Transaction on Election Device", Vol. ED-22, No. 7, pages 429-433, (July, 1975) referred to in Japanese Patent Examined Publication No. 58-26825 (WO No. 79/00340, PCT/US78/00179) teaches the necessity of filling the less-attenuation chamber with He or a very low pressure of air.
In FIG. 1, X-rays 2 generated from an X-ray source 1 including an electron beam generator 1a and an X-ray target 1b are entered into a less-attenuation chamber 5 through a vacuum chamber 12 and a beryllium window 4, irradiated on a mask 9, and reach a resist on a wafer 11 through a gap 10 between the mask 9 and the wafer 11 so that, the shape of a pattern made of X-ray absorbing material on the mask 9 is transferred onto the resist. If the X-rays were largely absorbed before reaching the resist, under-exposure would be caused as described above. However, the X-ray transmittivity K.sub.x is defined by the following equation and variable depending on the absorption coefficiency .mu. of the material and the passing distance t through which the X-ray is transmitted. EQU K.sub.x =exp (-.mu.t)
The value of .mu.is smaller as the atomic number of the material is smaller and the wavelength of the X-ray is shorter.
In the case where the wavelength of the X-rays 2 and the transmitting distance t are selected to be 5.4 .ANG. and 30 cm respectively, the X-ray transmittivity K.sub.x is almost 100% in the vacuum chamber 12, 98% in the beryllium window 4, almost 100% in the gap 10, and 95% in the less-attenuation chamber 5 filled with 100% helium. Thus, it is found that the X-ray transmittivity K.sub.x is the lowest in the less-attenuation chamber 5. Accordingly, it becomes necessary to make the X-ray transmittivity K.sub.x in the low attenuation chamber 5 as high as possible and to maintain the highest transmittivity K.sub.x stably.
In the prior art, as described above, a helium gas or the like is filled in the less-attenuation chamber 5 to prevent the X-ray transmittivity from lowering. In any case, however, no prior art is found which suggests to pay attention on stable maintenance of the X-ray transmittivity K.sub.x and teaches a specific technique for achieving such stable maintenance. In the prior art, therefore, there has been a disadvantage that it is difficult to judge whether a He gas is sufficiently substituted for atmospheric air in the less-attenuation chamber 5, whether a predetermined percentage of He gas is supplied, whether there is leakage of the He gas, or the like. Further, because He gas is expensive, it is economically undesired to supply extra He gas on the assumption that there occurs leakage of helium.